Turnable free space optical filters

ABSTRACT

An optical apparatus, comprising a semiconductor substrate, a dielectric layer located on the semiconductor substrate, wherein a membrane portion of the dielectric layer is located over a cavity in a surface of the semiconductor substrate, a resistive heater located on the membrane portion, the resistive heater being controllable by a current applied to the resistive heater and an etalon optical filter located on the resistive heater and over the cavity, an optical passband of the etalon optical filter being wavelength tunable by the resistive heater. A method of manufacturing the optical apparatus is also disclosed.

TECHNICAL FIELD

This application is directed, in general, to optical communicationsapparatuses and in particular, planar optical assemblies and theirmanufacture.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the inventions. Accordingly, the statements of thissection are to be read in this light and are not to be understood asadmissions about what is prior art or what is not prior art.

Tunable filters are important components of wavelength tunable opticalapparatuses such as wavelength-division multiplexing (WDM) receivers,optical system monitors and tunable lasers.

Tunable filters are often tuned by angular rotation, carrier injectionor thermal means. Mechanical angle tuning can require largeelectrostatic or electromagnetic motor arrangements. Carrier injectioncan introduce absorption losses together with refractive index tuning.Thermal tuning can be subject to tuning crosstalk from unintendedthermal variations from the environment or other co-integratedcomponents that are dissipating varying heat loads over time.

SUMMARY

One embodiment is an optical apparatus, comprising a semiconductorsubstrate, a dielectric layer located on the semiconductor substrate,where a membrane portion of the dielectric layer is located over acavity in a surface of the semiconductor substrate, and a resistiveheater located on the membrane portion, the resistive heater beingcontrollable by a current applied to the resistive heater. An etalonoptical filter of the apparatus is located on the resistive heater andover the cavity, an optical passband of the etalon optical filter beingwavelength tunable by the resistive heater.

Some embodiments of the etalon optical filter can include a silicon slabbeing upright on the dielectric layer.

Any such embodiments of the apparatus can further include another etalonoptical filter located on another membrane portion of the dielectriclayer over another cavity in the surface of the semiconductor substrate,and another resistive heater located below the another etalon filter andon the another portion of the dielectric layer. The another resistiveheater can be controllable by applying a current thereto and the anotheretalon optical filter can be wavelength tunable by the another resistiveheater.

In some embodiments the dielectric layer can be a silica layer and thesemiconductor substrate can be a silicon substrate. In some suchembodiments, the semiconductor substrate can be a silicon optical benchsubstrate.

In any such embodiments of the apparatus an area footprint of theresistive heater on the membrane portion can be within an area footprintof the etalon optical filter on the membrane portion.

In some embodiments of the apparatus the resistive heater can beelectrically connected to a metal filled via passing through thedielectric layer.

In some embodiments of the apparatus, the etalon optical filter can becoated with a partially reflective dielectric material layer.

In some embodiments of the apparatus, the cavity can have an undercutportion with an undercut length below the membrane portion that can be avalue in a range from 50 to 120 microns, and a maximal cavity depthbelow the membrane portion that can be in a range from 80 to 240microns.

Any such embodiments of the apparatus can include a thermally tunableoptical filter that is part of a planar optical assembly that furtherincludes a portion of an optical fiber positioned on semiconductorsubstrate to transmit a light of different selected wavelengths throughthe etalon optical filter. In some such embodiments, the optical fibercan be located to transmit the light via the etalon optical filter to aphotodetector located on the semiconductor substrate.

In any such embodiments, the planar optical assembly can further includea tunable optical phase shifter chip physically located on a differentone of the resistive heater layer and located between a first one of theetalon optical filter and a second one of the etalon optical filter,wherein the phase chip is thermally tunable by applying another currentthrough the different one of the resistive layer.

In any such embodiments, the planar optical assembly can further includea reflective semiconductor optical amplifier gain chip located on thesemiconductor substrate and optically located between the optical fiberand the thermally tuned optical filter.

In any such embodiments, the planar optical assembly can further includea lens located on the semiconductor substrate and between thephotodetector and the thermally tunable optical filter.

In any such embodiments, the planar optical assembly can further includea lens located on the semiconductor substrate and between the opticalfiber and a partial mirror or isolator located on the semiconductorsubstrate.

In any such embodiments, the planar optical assembly can be awavelength-tunable optical receiver for a Wavelength DivisionMultiplexing Passive Optical Network.

Another embodiment is method of manufacturing an optical apparatus. Themethod can include forming a thermally tuned optical filter, which caninclude providing a semiconductor substrate, depositing a dielectriclayer on the semiconductor substrate and forming a cavity in thesemiconductor substrate wherein a membrane portion of the dielectriclayer is located over the cavity in the semiconductor substrate. Themethod can include forming a resistive heater layer on the membraneportion and forming an electrode layer connected to the resistive layersuch that a temperature of the resistive layer is controllable by acurrent applied from the electrode layer to the resistive layer. Themethod can include positioning an etalon optical filter on the resistivelayer and over the cavity, where an optical passband through the etalonoptical filter is tunable by changing a refractive index of the etalonoptical filter from the temperature change of the resistive layer.

In any embodiments of the method, the forming of the cavity can include,after forming the resistive layer and after forming the electrode layer,forming one or more openings in the dielectric layer and then etchingthe semiconductor substrate through the one or more openings.

In any embodiments of the method, the forming of the resistive layer andthe forming of the electrode layer can include, after forming thecavity, sputter depositing a nickel-chromium layer on the dielectriclayer and then patterning the nickel-chromium layer to form theresistive layer and the electrode layer connected to the resistivelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the disclosure are best understood from the followingdetailed description, when read with the accompanying FIGUREs. Somefeatures in the figures may be described as, for example, “top,”“bottom,” “vertical” or “lateral” for convenience in referring to thosefeatures. Such descriptions do not limit the orientation of suchfeatures with respect to the natural horizon or gravity. Variousfeatures may not be drawn to scale and may be arbitrarily increased orreduced in size for clarity of discussion. Reference is now made to thefollowing descriptions taken in conjunction with the accompanyingdrawings, in which:

FIG. 1A presents a perspective view of an example embodiment of anoptical apparatus of the disclosure;

FIG. 1B presents a perspective view of another example embodiment of anoptical apparatus of the disclosure;

FIG. 1C presents a detailed side view of a portion of the apparatusdepicted in FIG. 1B;

FIG. 1D presents a top down view of another example embodiment of anoptical apparatus of the disclosure, similar to the optical apparatusesdepicted in FIGS. 1A-1C;

FIG. 1E presents a side view of the optical apparatus of depicted inFIG. 1D;

FIG. 2A presents a side view an example planar optical assembly andwhich can include any embodiments of the apparatus disclosed in thecontext of FIGS. 1A-1E;

FIG. 2B presents a top down view of the optical apparatus depicted inFIG. 2A;

FIG. 3A presents another example embodiment of the optical apparatussimilar to the view shown in FIG. 2B;

FIG. 3B presents another example embodiment of the optical apparatussimilar to the view shown in FIG. 3A;

FIG. 4 presents a flow diagram illustrating selected steps in an examplemethod of manufacturing an optical apparatus of the disclosure includingany of the apparatus embodiments disclosed in the context of FIGS.1A-3B;

FIG. 5 presents example results for a test apparatus showing therelationship between etch undercut and etch depth of a cavity in asilicon embodiment of the high thermal conductance layer, as a functionof a mask area opening in a glass layer embodiment of a silica lowthermal conductance layer;

FIGS. 6A and 6B present example plots of (A) measured filter refractiveindex change due to temperature change (squares; linear fit, dashedline), and, (B), filter optical transmission response to heaterelectrical power;

FIG. 7 presents an example simulated composite external cavity filterspectrum showing wide bandwidth feedback selectivity for a test opticalassembly such as disclosed in the context of FIGS. 2A-2B;

FIGS. 8A and 8B present example plots of simulated thermal crosstalk ofphase tuning heater to etalons inducing frequency shift (A) andsimulated thermal crosstalk of RSOA bias to etalons (B) indicatingminimal disturbance of cavity filter response to gain and phase tuningof laser; and

FIG. 9 present example plots of measured thermal crosstalk of change ofelectrical power dissipation in a reflective semiconductor opticalamplifier chip coupled via etalon optical filters to an optical fibershowing induced optical frequency shift (a) and output optical power infiber (b) indicating minimal disturbance of etalon and cavity filterresponse to gain tuning of laser.

In the Figures and text, similar or like reference symbols indicateelements with similar or the same functions and/or structures.

In the Figures, the relative dimensions of some features may beexaggerated to more clearly illustrate one or more of the structures orfeatures therein.

Herein, various embodiments are described more fully by the Figures andthe Detailed Description. Nevertheless, the inventions may be embodiedin various forms and are not limited to the embodiments described in theFigures and Detailed Description of Illustrative Embodiments.

DETAILED DESCRIPTION

The description and drawings merely illustrate the principles of theinventions. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theinventions and are included within its scope. Furthermore, all examplesrecited herein are principally intended expressly to be for pedagogicalpurposes to aid the reader in understanding the principles of theinventions and concepts contributed by the inventor(s) to furthering theart, and are to be construed as being without limitation to suchspecifically recited examples and conditions. Moreover, all statementsherein reciting principles, aspects, and embodiments of the inventions,as well as specific examples thereof, are intended to encompassequivalents thereof. Additionally, the term, “or,” as used herein,refers to a non-exclusive or, unless otherwise indicated. Also, thevarious embodiments described herein are not necessarily mutuallyexclusive, as some embodiments can be combined with one or more otherembodiments to form new embodiments.

Embodiments of the invention are driven by the desire to increase levelsof integration for optical assemblies expected to have increasingthermal densities and gradients. The invention embodiments provide anoptical assembly with improved thermal isolation of etalon opticalfilters. The assembly integrates a thermal isolation layer so that theoptical filters are located on the layer and over a free space cavity inthe substrate of the assembly. Other assembly components (e.g.,detectors and gain chips) can thereby be thermally tuned whilesimultaneously being thermally isolated from power dissipatingcomponents on the same substrate.

As demonstrated herein these features also facilitate the manufacture ofoptical apparatus embodiments, such as optical assemblies, with reducedthermal crosstalk effects during tuning and consequently there isreduced complexity for adjusting filter and phase controls. Suchadjustability can be over substantial parts of a laser power curve, withthe lasing frequency unaffected by the large changes in a reflectivesemiconductor optical amplifier gain chip (RSOA) bias power.

One embodiment of the disclosure is an optical apparatus, FIGS. 1A and1B presents a perspective view of an example embodiments of an opticalapparatus 100 of the disclosure and FIG. 1C presents a detailed sideview of a portion of the apparatus depicted in FIG. 1B. FIG. 1D presentsa top down view of another example embodiment of an optical apparatus ofthe disclosure, similar to the optical apparatuses depicted in FIGS.1A-1C and FIG. 1E presents a side view of the optical apparatus ofdepicted in FIG. 1D.

One embodiment of the disclosure is an optical apparatus. FIGS. 1A and1B present perspective views of example embodiments of an opticalapparatus 100 of the disclosure. FIG. 1C presents a detailed side viewof a portion of the apparatus depicted in FIG. 1B. FIG. 1D presents atop down view of another example embodiment of an optical apparatus ofthe disclosure, i.e., similar to the optical apparatus depicted in FIGS.1A-1C. FIG. 1E presents a side view of the optical apparatus of depictedin FIG. 1D.

Embodiments of the dielectric layer 110 can be or include a low thermalconductance layer that can be made of silica (e.g., silicon dioxide,thermal conductance equal to about 1.4 W/mK at ° C.), silicon nitride 6W/mK at 20° C., aerogel (0.03 W/mK at 20° C.) or similar materialsfamiliar to those skilled in the pertinent art, such as materials havinga thermal conductivity value in a range from 0.01 to 10 W/m K at 20° C.,or an upper thermal conductivity value of less than 20 W/mK at 20° C. insome embodiments.

Embodiments of the semiconductor substrate 105 can be or include a highthermal conductance layer that can be made of silicon (130 W/mK at 20°C.), gallium arsenide (52 W/mK at 20° C.), indium phosphide (68 W/mK at20° C.) or similar materials familiar to those skilled in the pertinentart, such as materials having a thermal conductivity value in a rangefrom 50 to 150 W/m K at 20° C., or an lower thermal conductivity valueof greater than 50 W/mK at 20° C. in some embodiments.

In some embodiments, to facilitate thermal isolation of the etalonoptical filter, the dielectric layer 110 (e.g., low thermal conductancelayer) has a thermal conductance that is at least 10 (and in someembodiments, at least 20, 40, 60, 80, 100, 500, 1000 or 5000) or timeslower than the thermal conductance of the semiconductor high thermalconductance layer.

Some embodiments of the resistive heater 130 can be or include a layercomposed of chrome (e.g., chromium, resistivity equal to about 4e-6 Ω·mat 20° C.), nickel-chromium (1.1e-6 Ω·m at 20° C.), tantalum nitride(2.0e-6 Ω·m at 20° C.) or similar materials familiar to those skilled inthe pertinent art, such as materials having a resistivity value in arange from 0.5 to 5 e-6 Ω·m at 20° C., or a minimum resistivity value ofequal to or greater than 0.2 e-6 Ω·m in some embodiments. In someembodiments, the resistive heater 130 includes a nickel chromium alloylayer.

Some embodiments of the etalon optical filter can be or include highthermal conductance and low optical loss materials such as silicon,gallium arsenide or indium phosphide or similar materials familiar tothose skilled in the pertinent art, such as materials providing an upperoptical loss value of the etalon of less than 0.2 dB in someembodiments.

Some embodiments of the membrane portion 120 can include or bedielectric low thermal conductance layer having a thickness 145 in arange from 2 to 20 microns (e.g., 12 micron).

As illustrated, embodiments of the etalon optical filter 140 can includea silicon slab being upright on the dielectric layer 110.

Embodiments of the apparatus 100 can include another etalon opticalfilter 140 b located on another membrane portion 120 b of the dielectriclayer 110 over another cavity 125 b in the surface 107 of thesemiconductor substrate 105, and another resistive heater 130 b locatedbelow the another etalon filter 140 b and on the another portion 120 bof the dielectric layer 110. The other resistive heater 130 can becontrollable by applying a current thereto and the other etalon opticalfilter 140 b can be wavelength tunable by the another resistive heater130 b.

For instance, the optical apparatus 100 can include one or morethermally-tuned optical filters 102. For example each thermally-tunedoptical filter 102 can include the dielectric low thermal conductancelayer 110 located on a semiconductor high thermal conductance layer 105,where a membrane portion 120 of the low thermal conductance layer islocated over a cavity 125 in the high thermal conductance layer, theresistive heater 130 located on the membrane portion where a temperatureof the resistive layer is controllable by a current applied from anelectrode layer 132 connected to the resistive layer and the etalonoptical filter 140 located on the resistive layer and over the cavity,where the optical passband through the etalon optical filter is tunableby changing a refractive index of the etalon optical filter from thetemperature change of the resistive heater layer.

For instance, to provide enhanced composite optical tuning, theapparatus 100 can include two or more of the thermally tunable opticalfilters 102, 102 b each having two or more of the etalon optical filter140 a, 140 b located on different ones of the resistive heater 130 a,130 b which are located as layers on different ones of the membraneportions 120 a, 120 b and which are located over different ones of thecavities 125 a, 125 b in the semiconductor high thermal conductancelayer 110, where the different ones of the cavities are separated by apillar portions 150 of the semiconductor high thermal conductance layer105, the pillar portions contacting the dielectric (e.g., low thermalconductance) layer 110.

In some embodiments of the apparatus 100, the dielectric layer 110 is asilica layer and the semiconductor substrate 105 is a silicon substrate.In some such embodiments, the semiconductor substrate 105 is a siliconoptical bench substrate.

For instance, for some embodiment, the dielectric layer 110 (e.g., lowthermal conductance layer) can include a silica layer and thesemiconductor substrate 105 (e.g., high thermal conductance layer) caninclude a doped and annealed silicon layer located on a wafer substrate160. In some such embodiments the wafer substrate 160 can be our includea silicon optical bench substrate. In some such embodiments, the wafersubstrate (e.g., substrate 160) preferably has a thermal expansioncoefficient that is the same or nearly the same (e.g., within 0.01, 0.1,1, 2, 5 or 10%) as the semiconductor high thermal conductance layer 105.

As illustrated in FIG. 1B in some apparatus embodiments an areafootprint 161 of the resistive heater 130 a on the membrane portion 120a can be within an area footprint 163 of the etalon optical filter 140 aon the membrane portion 120 a. For instance, in some embodiments an areafootprint 161 of the resistive heater 130, in a plane 162 of themembrane portion 120 is less than and within an area footprint 163 ofthe etalon optical filter 140. As a non-limiting examples, the areafootprint 163 of the etalon optical filter can have a value in a rangefrom 250×250 to 1000×1000 squared microns, and in some embodiments,300×500 squared microns, and, the area footprint 161 of the resistivelayer can have a value in a range from 3×3000 squared microns to 100×100squared microns and in some embodiments, 30×300 squared microns.

In some embodiments, to provide an alternate means to supply power tothe resistive heater (130), a power supply source 167 is electricallyconnected to a metal filled via 168 passing through the dielectric layer110. For instance, in some embodiments, the electrode layer 132 can beconnected to an electrically conductive metal filled via 168 passingthrough the dielectric layer 110 and semiconductor substrate 105 to anunderlying power source 167 located on a wafer substrate 160.

In any embodiments of the apparatus, the etalon optical filter can becoated with a partially reflective material layer 170. For instance,partially reflective material layer serving as Bragg coating layers 170of the etalon optical filter 140 can have a thickness 175 in dimensionperpendicular to a surface 107 of the dielectric low thermal conductancelayer 110, that are equal to about one-quarter wavelength of light(e.g., C and L bands; e.g., 1550 nm) directable through the etalonoptical filter.

For instance, a pair of silica and silicon coatings, serving aspartially reflective material layers 170, of approximately quarter waveoptical thickness at 1550 nm can be coated on both sides of a siliconwafer and the wafer diced to provide one-pair Bragg mirrors withapproximately 82% reflectivity per side. One skilled in the art wouldappreciate how the etalon optical filter's thickness (e.g., a value in arange from 5 to 200 microns for some embodiments) in the optical pathdirection (e.g., FIG. 2A direction of transmitted light 210) would bedecided by the filter's target free spectral range FSR and opticalindex.

As illustrated in FIG. 1C, some embodiments of the cavity 125 have anundercut portion 180 with an undercut length 182 below the membraneportion 120 that is a value in a range from 50 to 120 microns, and amaximal cavity depth 184 below the membrane portion 120 can be in arange from 80 to 240 microns. In some embodiments, the undercut portion180 of the dielectric layer 110, and in embodiments, the entiredielectric lay 110, has a thickness 186 that is a value in a range from5 to 20 microns; e.g., 12 microns in some embodiments. Such undercuttingcan further help in thermally isolating the etalon from the bulk of thehighly thermally conductive semiconductor layer/substrate 150.

Some embodiments of the optical apparatus are part of a planar opticalassembly.

FIG. 2A presents a side view of an example planar optical assembly 200,which can include any embodiments of the apparatus 100 disclosed in thecontext of FIGS. 1A-1E. FIG. 2B presents a top down view of the opticalapparatus depicted in FIG. 2A. FIG. 3A presents another exampleembodiment of an optical apparatus similar to that shown in FIG. 2B.FIG. 3B presents another example embodiment of an optical apparatussimilar to that shown in FIG. 3A.

With continuing reference to FIGS. 1A-3B throughout, some embodiments ofthe apparatus 100 include the thermally tunable optical filter 102 thatis part of a planar optical assembly 200 (FIGS. 2A and 2B) that canfurther include a portion of an optical fiber 205 (e.g., a fiber endsegment). The thermally tunable optical filter 102 and optical fiber 205can be positioned on the semiconductor substrate 105 to transmit a light210 of different selected various wavelengths through the etalon opticalfilter (e.g., one or more filters 140 a, 140 b). In some suchembodiments, the optical fiber 205 can be located to transmit a portionof the light 210 via the etalon optical filter to a photodetector 215(e.g., a solid state photon counting detector) located on thesemiconductor substrate 195 (e.g., high thermal conductance layer).

In some embodiments, the planar optical assembly 200 can further includea tunable optical phase shifter chip 220 physically located on adifferent resistive heater 230 and located between a first one of theetalon optical filter 140 a and a second one of the etalon opticalfilter 140 b, wherein the chip 220 is thermally tunable by applyinganother current through the different resistive heater 230

In some embodiments, the planar optical assembly 200 can further includea reflective semiconductor optical amplifier gain chip (RSOA) 240located on the semiconductor substrate 105 and optically located betweenthe optical fiber 205 and the thermally tunable optical filter 140 a.

In some embodiments, the planar optical assembly 200 can further includea lens 250 located on the semiconductor substrate 105 and between thephotodetector 215 and the thermally tunable optical filter 102 a.

In some embodiments, the planar optical assembly 200 can further includeincludes a lens 252 located on the semiconductor substrate 105 andbetween the optical fiber 205 and a partial mirror 255 or isolator 260located on the semiconductor substrate 105. Some embodiments can furtherinclude both such lens 250, 252 located as disclosed above.

In some embodiments, the planar optical assembly 200 can be or include awavelength-tunable optical receiver for a Wavelength DivisionMultiplexing Passive Optical Network.

In some embodiments, the planar optical assembly 200 can be or include astandalone tunable receiver further including first optical fiber (e.g.,fiber 205 a, FIG. 3A) to direct light through the filter 140, such asdiscloses elsewhere herein, to a second optical fiber (e.g., fiber 205b, FIG. 3A), or, to a detector 265 (e.g., a photodetector FIG. 3B)positioned on the semiconductor substrate 105 to receive the light 210passing through the etalon optical filter 140.

In some embodiments, the planar optical assembly 200 is a laser whoselaser cavity is between the reflector of the RSOA 240 and the partiallyreflective mirror 255. In such embodiments one or more etalon opticalfilters 140 a, 140 b are located to filter light in an optical lasercavity of the laser.

Another embodiment is a method of manufacturing an optical apparatus.FIG. 4 presents a flow diagram illustrating selected steps in an examplemethod 400 of manufacturing an optical apparatus of the disclosureincluding any of the apparatus 100 or assembly 200 embodiments disclosedin the context of FIGS. 1A-3B.

With continuing reference to FIGS. 1A-4 throughout, embodiments of themethod can include forming (step 410) a thermally tunable optical filter102. Forming the filter 102 can include providing (e.g., a first step415) a semiconductor substrate 105 (e.g., any embodiments of the highthermal conductance layer disclosed herein) and depositing (e.g., thenstep 420) a dielectric layer 110 (e.g., any embodiments of the lowthermal conductance layer disclosed herein) on the semiconductorsubstrate. In some embodiments depositing the dielectric layer (step420) includes a PECVD process, e.g., depositing layer of PECVD dopedsilica and then annealing so that dielectric layer is stress matched tothe semiconductor substrate.

Forming the thermally tunable optical filter 102 can also includeforming a cavity 125 (step 430) in the semiconductor substrate where amembrane portion 120 of the dielectric layer 110 will be located overthe cavity in the semiconductor substrate. In some embodiments, formingthe cavity (step 430) can include patterning and etching openings in thedielectric layer using anisotropic reactive ion etching (RIE). Forinstance (e.g., silica glass) anisotropic etching materials can beselected such that etching terminates with high selectivity on theunderlying semiconductor substrate. The forming step 430 can includethen, isotropically etching the semiconductor substrate 115, e.g., bysubjecting the semiconductor substrate to an appropriate isotropicetchant (e.g., SF₆) through the previously produced openings in thedielectric layer 110.

Forming the filter 102 can further include forming (step 440) aresistive heater 130 on the membrane portion 125 of the dielectric layer110 and forming (step 450) an electrode layer 132 connected to theresistive heater 130 such that a temperature of the resistive heater iscontrollable by a current applied by the electrode layer to theresistive heater.

Forming the filter 102 can further include positioning (step 450) anetalon optical filter 140 to be located on (e.g., directly on) theresistive heater and on the membrane portion of the dielectric layer andover the corresponding cavity, wherein an optical passband through theetalon optical filter is tunable by changing a refractive index of theetalon optical filter from the temperature change due to heat appliedthereto with the resistive heater.

In some such embodiments, forming the cavity (step 430) includes, afterforming 440 the resistive heater 130 (step 440) and after forming 450the electrode layer 132, forming one or more openings in the dielectriclayer 110 and then etching (e.g., isotropically etching) thesemiconductor substrate through the one or more openings.

In some such embodiments, forming the resistive heater (step 440) andthe forming of the electrode layer (step 450) can include, after formingthe cavity (step 430), sputter depositing a nickel-chromium layer (e.g.,resistive heater layer 130) on the dielectric layer 110 and thenpatterning the nickel-chromium layer to form the resistive layer and theelectrode layer connected to the resistive layer. In some embodimentsforming the resistive layer can include a lift-off process usingevaporation of the NiCr onto pre-patterned photoresist.

In some such embodiments, the positioning (step 460) of the etalonoptical filter the resistive heater 130 and over the cavity 125 includesa pick and place process wherein the etalon optical filter 140 ispositioned (e.g., as a vertical silicon slab) on an adhesive placed onthe resistive heater.

Experiments

Test optical apparatus assembly structures were manufactured and testedas further disclosed below.

A silica layer (e.g., silica low thermal conductance dielectric layer110, “silica”) was deposited on a silicon wafer (e.g., siliconsemiconductor high thermal conductance layer 105, “wafer”) by PECVD toform a doped silica layer which was then thermally annealed so that thelayer was stress matched to the silicon wafer. Openings in the silicalayer were formed by patterning and etching with etching terminated withhigh selectivity on the underlying silicon wafer. A metal resistiveheating layer (e.g., resistive heater 130, “heater”) was deposited usingsputtering of nickel-chromium. A thin encapsulation dielectric wasdeposited and vias were opened to the underlying heater. An electrodelayer was then deposited to connect the heater and to provide wiring fora gain chip, thermistor and monitor photodiode. A membrane portion ofthe silica layer was formed by a reactive ion etching (SF₆ RIE) to forma cavity in the underlying portion of the silicon wafer. Ball lenscavities in the silicon wafer were also formed using SF₆ RIE.

Cross-sections of test etched cavities in test apparatuses, analogous tothe cross-section shown in FIG. 1C, were fabricated and quantified. Thedimensions of the membrane were determined based on the thickness of theoptical etalon filter needed for a robust laser tuning design and therequirements of the pick-and-place equipment used to attach the filtersto the membranes after fabrication. As shown in FIG. 5 , the siliconunder-etch was not fully isotropic, with the etch depth (D) and undercut(U) varying depending on the relative mask opening area due to thedynamics of etchant gas propagation into the cavities. The series oftest apparatus structures were fabricated with cavities to identifyscaling rules and to ensure robustness to fabrication processesincluding wafer spinning and dicing. The resulting dependence of D and Ubased on mask opening area was measured and fitted to facilitate anappropriate design for an efficiently tunable etalon filter with robustmechanical properties.

Informed by the above analysis, test optical assemblies that includedembodiments of the optical apparatus were built. The etalon opticalfilter dimensions was 140 microns thick, 500 microns wide and 300microns tall. A resistive layer (resistive heater 130) of approximately10 squares of 36 Ω/square NiCr fitting between two silicon pillars wasplaced 400 microns center-to-center. This design facilitated heatgenerated in the resistive layer to be directed to travel into theetalon optical filter and then to the edges to reach the silicon pillarpath to a thermal ground of the substrate (silicon optical benchsubstrate, SiOB). Since the silicon etalon optical filter's conductivitywas about 100 times larger than the silica low thermal conductance layerand membrane, the filter was uniformly heated with substantially novariations in temperature across the filter sufficient to distort tuningbehavior.

Silicon etalon optical filters were fabricated by depositing a pair ofsilica and silicon coatings of approximately quarter wave opticalthickness at 1550 nm on both sides of a silicon wafer. The resultingfilms provide one-pair Bragg mirrors with approximately 82% reflectivityper side. Because of the thermal isolation design as disclosed herein,such coated silicon wafers could be used as etalon optical filterswithout further processing, thereby avoiding the need for integratingwith complex heating structures. The ability to avoid such furtherprocessing would beneficially improve production yield since such thinwafers (e.g., 140 microns thick) are fragile and prone to damage. Theoptical spectrum of the etalon optical filters was measured usingoptical collimators and the resulting data was fitted to extractestimated losses and Finesse. Insertion losses of about 0.6 dB loss atpeak, a 303 GHz free-spectral range and filter Finesse of about 14.5were measured.

The silica coated silicon wafers were diced into 300 micron by 500micron chips to form the etalon optical filters for assembly on a SiOBsubstrate to form the test optical assembly having a single filter.

Light was coupled from an input optical fiber through the filter andinto an output fiber by ball lenses that were actively aligned and thenfixed in place with epoxy. The filter was tested by using athermoelectric cooler (TEC) to increase the temperature of the entireassembly and measure the resulting change in the transmitted opticalpower. The thermistor used to provide the TEC control loop feedback wasembedded in the aluminum base under the sample. The tuning range wassmall enough that there was a small offset between the recordedtemperature of the thermistor and the filter. The optical spectrum as afunction of temperature was measured and then, by fitting the shift ofthe etalon response, the thermo-optic coefficient of the filter materialwas determined FIG. 6A to have a value 1.80 per ° C. Based on this atemperature change of 29° C. was estimate to provide tuning over onefull free spectral range (FSR).

In a further test, the TEC was used to fix the assembly temperature to25° C. and current was applied to the resistive layer to heat andthereby tune the filter. The applied current and measured voltage acrossthe resistive layer was used to derive the dissipated electrical powerand the change in transmitted optical power from a distributed feedbacklaser source (DFB, 1531 nm) was measured over a range of powers. FIG. 6Bshows the measured filter tuning as a function of electrical power(heating power). Tuning over one full FSR was obtained with only 25.3 mWof injected electrical power due to the thermal isolation as disclosedherein.

Further testing was performed on an optical assembly with the apparatusincluding two etalon optical filters to achieve a wide frequency rangeof tuning. An assembly such as shown in FIG. 2A-2B was fabricated.

A silicon phase chip (e.g., optical phase shifter chip 220) was locatedon membrane portions (e.g., membrane portions 120) and between first andsecond filters (e.g., filters 140 a, 140 b) to facilitate providingphase alignment of the composite tuning filter with the overall lasercavity modes and activated by the same thermal tuning mechanism as thefilters. The phase chip had antireflection (AR) coatings on both facetsto help prevent unwanted frequency ripples. The phase tuner wasfabricated from 700 micron thick silicon wafers to simplify handling andso a larger membrane portion was employed below it. The ReflectiveSemiconductor Optical Amplifier (e.g., RSOA gain chip 240) was mounteddirectly on SiOB substrate with a portion of the 12 micron silica lowthermal conductance layer removed for efficient thermal transfer to theTEC located on the back side of the substrate. A glass partial mirror(e.g., partial mirror 255) chip coated for 80% transmission and 20% backreflection was used to close the optical path of the laser cavity. Afterthe back reflector (partial mirror), outside the laser cavity, a latchedgarnet optical isolator (e.g., isolator 260) was added before the outputoptical fiber (e.g., fiber 205) to help prevent parasitic externalreflections from entering the laser cavity. Two ball lenses (e.g., lens250, 252) were used to couple light (e.g., light 210) from the RSOA tothe fiber.

The physical path length of the composite laser cavity was about 4 mmwhich was equivalent to approximately 9.7 mm in free space (neglectingetalon optical filter resonant enhancement). The long optical cavity isthought to be beneficial for linewidth reduction for coherentcommunication purposes where low phase noise is critical forhigh-capacity modulation. The assembly size was 11×5 mm, but, since thecomponents were relatively broadly spaced to ensure straightforwardassembly for testing purposes, assemblies of reduced sizes can be made.

The FSR values of the two etalon optical filters were selected to yielda wide composite FSR (˜100 nm) for C+L band tuning. The etalon opticalfilters were fabricated from silicon wafers of about 140 micronsthickness and two selected portions of the wafer were with appropriatethicknesses to provide the etalon optical filters. The calculatedcomposite cascaded filter spectrum of the vernier pair is shown in FIG.7 and is based on the measured single Bragg pair mirror Finesse. Thetotal extended free spectral range was 12.6 THz and the excess loss ofthe filter from the main peak to the next highest peak was 3.3 dB toensure stable lasing. The cavity mode spacing estimated to be about 6GHz. The composite frequency filtering response was sufficiently narrowgiven the filter Finesse to select a single cavity mode for stablelasing.

Further testing including simulating the complete laser geometry topredict the thermal crosstalk between each etalon optical filter and thephase tuner and RSOA as a function of power dissipated in each element.Crosstalk between tuning and optical power controls is generallyexpected to cause calibration complexity in traditional photonicintegrated tunable lasers.

The effect of thermal crosstalk from the phase tuning chip to thefilters was simulated. Since the phase must be adjusted after anyfrequency tuning the crosstalk of the filters to the phase chip was notrelevant. The temperature rise in each filter was calculated based onthe heater power dissipated under the phase chip. The filter crosstalkterms were essentially symmetric due to the layout of the assembly onthe SiOB substrate. The filter temperature rise was converted to aresulting etalon optical filer frequency shift as shown in FIG. 8A. Overthe full phase tuning range, negligible filter detuning due to crosstalkof under 100 MHz is expected and the required laser control precisionwas ±1.5 GHz. The low detuning due to crosstalk is attributed to thehigh thermal insulation of the membrane portions located under both ofthe filters as disclosed herein.

To experimentally demonstrate the successful minimization of lasercalibration and control crosstalk scanned the tunable laser RSOA biaswere scanned from threshold to full output. The RSOA bias power variedby 600 mW across this range.

It is generally expected, for both monolithic and hybrid waferintegrated lasers, there to be substantial thermal and/or electricalcrosstalk effect during such tuning. Consequently, it is expected thatsubstantial adjustment of both filter and phase controls will benecessary during optical power adaptation to avoid undesired modehopping or other shifts from the target frequency.

First, the laser was tuned to 197 THz at 350 mA RSOA bias based on agenerated tuning map. This aligned the etalon and cavity modes togetherat the target frequency. Then the RSOA bias was swept from 80 mA, justabove threshold, to 360 mA while adjusting only the phase tuning powerat each step to realign the laser cavity mode after the change incarrier density of the RSOA has shifted the effective optical pathlength. The resistive layer heater power to the optical etalon filterswas not adjusted.

The resulting laser output power and frequency response, plotted in FIG.9 , shows that over the substantial part of the laser power curve, thelasing frequency was unaffected by the large changes in RSOA bias power.As an example, if the output power adjustment over lifetime tocompensate for gain chip aging is 1 dB the RSOA bias would be adjustedby approximately 100 mA. The significant degree of tunable lasercalibration and control simplification is attributed to the thermalengineering of the optical design and assembly as disclosed herein.

Although the present disclosure has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the scope ofthe invention.

1. An optical apparatus, comprising: a semiconductor substrate; adielectric layer located on the semiconductor substrate, wherein amembrane portion of the dielectric layer is located over a cavity in asurface of the semiconductor substrate; a resistive heater located onthe membrane portion, the resistive heater being controllable by acurrent applied to the resistive heater; and an etalon optical filterlocated on the resistive heater and over the cavity, an optical passbandof the etalon optical filter being wavelength tunable by the resistiveheater.
 2. The optical apparatus of claim 1, wherein the etalon opticalfilter includes a silicon slab being upright on the dielectric layer. 3.The optical apparatus of claim 1, further including another etalonoptical filter located on another membrane portion of the dielectriclayer over another cavity in the surface of the semiconductor substrate;and another resistive heater located below the another etalon filter andon the another portion of the dielectric layer, wherein the anotherresistive heater is controllable by applying a current thereto; and theanother etalon optical filter being wavelength tunable by the anotherresistive heater.
 4. The optical apparatus of claim 1, wherein thedielectric layer is a silica layer and the semiconductor substrate is asilicon substrate.
 5. The optical apparatus of claim 4, wherein thesemiconductor substrate is a silicon optical bench substrate.
 6. Theoptical apparatus of claim 1, an area footprint of the resistive heateron the membrane portion is within an area footprint of the etalonoptical filter on the membrane portion.
 7. The optical apparatus ofclaim 1, wherein the resistive heater is electrically connected to ametal filled via passing through the dielectric layer.
 8. The opticalapparatus of claim 1, wherein the etalon optical filter is coated with apartially reflective dielectric material layer.
 9. The optical apparatusof claim 1, wherein the cavity has an undercut portion with an undercutlength below the membrane portion that is a value in a range from 50 to120 microns, and a maximal cavity depth below the membrane portion thatis in a range from 80 to 240 microns.
 10. The optical apparatus of claim1, wherein the apparatus includes a thermally tunable optical filterthat is part of a planar optical assembly, the planar optical assemblyfurther including a portion of an optical fiber positioned on thesemiconductor substrate to transmit a light of different wavelengthsthrough the etalon optical filter.
 11. The optical apparatus of claim10, wherein the optical fiber is located to transmit the light via theetalon optical filter to a photodetector located on the semiconductorsubstrate.
 12. The optical apparatus of claim 10, wherein the planaroptical assembly further includes a tunable optical phase shifter chipphysically located on a different resistive heater and located betweenthe etalon optical filter and another etalon optical filter, wherein thephase chip is thermally tunable by applying another current through thedifferent resistive heater.
 13. The optical apparatus of claim 10,wherein the planar optical assembly further includes a reflectivesemiconductor optical amplifier gain chip located on the semiconductorsubstrate and optically located between the optical fiber and thethermally tuned optical filter.
 14. The optical apparatus of claim 10,wherein the planar optical assembly further includes a lens located onthe semiconductor substrate and between the photodetector and thethermally tunable optical filter.
 15. The optical apparatus of claim 10,wherein the planar optical assembly further includes a lens located onthe semiconductor substrate and between the optical fiber and a partialmirror or isolator located on the semiconductor substrate.
 16. Theoptical apparatus of claim 10, wherein the planar optical assembly is awavelength-tunable optical receiver for a Wavelength DivisionMultiplexing Passive Optical Network.
 17. The optical apparatus of claim3, further comprising a laser, wherein two of the optical etalon filtersare located to filter light in an optical laser cavity of the laser. 18.A method of manufacturing an optical apparatus, comprising: forming athermally tuned optical filter, including: providing a semiconductorsubstrate; depositing a dielectric layer on the semiconductor substrate;forming a cavity in a surface of the semiconductor substrate wherein amembrane portion of the dielectric layer is located over the cavity inthe surface of the semiconductor substrate; forming a resistive heaterlayer on the membrane portion and forming an electrode layer connectedto the resistive layer such that a temperature of the resistive layer iscontrollable by a current applied from the electrode layer to theresistive layer; and positioning an etalon optical filter on theresistive layer and over the cavity, wherein an optical passband throughthe etalon optical filter is tunable by changing a refractive index ofthe etalon optical filter from the temperature change of the resistivelayer.
 19. The method of claim 18, wherein the forming of the cavityincludes, after forming the resistive layer and after forming theelectrode layer, forming one or more openings in the dielectric layerand then etching the semiconductor substrate through the one or moreopenings.
 20. The method of claim 18, wherein the forming of theresistive layer and the forming of the electrode layer includes, afterforming the cavity, sputter depositing a nickel-chromium layer on thedielectric layer and then patterning the nickel-chromium layer to formthe resistive layer and the electrode layer connected to the resistivelayer.